Does the company engage a pension consultant to inform and (to some extent) advise employees on their pension choices?
Have you sought your own independent, professional advice?
If you don't already own your own home and plan to some day (soon) then it might make sense to priortise saving for that but to contribute as much to the pension as you need to gain full advantage of the maximum employer contribution (I presume that they will match employee contributions up to the 5% limit?).
Apart from that check the charges on the scheme to see that they are reasonable - how much of each contribution goes in bid-offer spread, commissions, what allocation rate, what monthly charges - e.g. policy fee, what annual management charge.
In terms of fund selection forget about past performance and with in or around 4 decades to go to retirement you should probably think about going for a high equity/high risk-reward fund (e.g. 100% equities) rather than anything more conservative to maximise your returns over that period.
When you get closer to retirement you can move into more conservative funds/investments to lock in gains (assuming you are going to take the pension at retirement and not going to take your 25% tax free lump sum and/or roll over into a further pension investment such as an
Approved [Minimum] Retirement Fund etc.).
Investing in a 100% equity fund with a high risk/reward profile will mean volatility/fluctuations in value over time but you have decades to ride this out and you should not worry unduly about it. Hope this helps.
Try the
Pensions Board website for more general info on pensions.